The bleaching of sulphate pulp is divided into initial and final bleaching. During the initial bleaching, most of the lignin present in the pulp is removed. In the final bleaching, the residual lignin still present in the pulp is removed, and pulp darkening coloured groups, chromophores, are converted into the non-light-absorbing form.
Conventionally the initial bleaching consists of two stages: an acid delignification stage such as a chlorine dioxide stage D0 followed by a washing step, and an alkaline extraction stage E, which is often reinforced with oxygen and peroxide (EOP) or one of these (EO or EP). Between the D0 and E stages the pulp is washed. Said first acid delignification stage removes a main part of metals present in the pulp as well as hexenuronic acids consuming bleaching chemicals. The reactions of chlorine dioxide in the D0 stage are rapid. Although the dosage of an active agent in the D0 stage is often relatively high, a main part of the charged chemicals is consumed in a few seconds. However, the retention time in the D0 stage is usually about 30 minutes to ensure the reaction of all chemicals and to achieve a kappa number after the D0 EOP stages being as low as possible.
The reactions of chlorine dioxide degrade the lignin structures. A part of lignin is removed in the washing step of the D0 stage, but a substantial part of the reacted lignin will be converted into a dissolving form only in a following alkaline stage. Therefore, the filtrate resulting from the E stage contains a remarkable amount of dissolved organic matter as well as chlorine bound to the lignin during the D0 stage.
When an effluent cycle of a bleaching process is closed, the amount of effluents to be discharged has been tried to reduce by using filtrates in other processes of the mill, i.a. for washing of brown stock. Thereby the filtrates and the dissolved wood material and chemicals present in the filtrate are led to a recovery process of chemicals. The filtrates resulting from the D0 stage contain a main part of chlorides which are detrimental to a recovery process. Therefore, there has been efforts primarily to recover and circulate in the process filtrates resulting from the alkaline stage of bleaching. However, also the filtrates resulting from the alkaline stage subsequent to the D0 stage contain chlorine compounds.
When using peroxide, the transition metals, such as Mn, Fe and Cu, are removed or their content is reduced prior to a peroxide stage. An advantageous metal profile in the peroxide stage decreases the degradation of a bleaching perhydroxide ion —OOH into oxygen radicals. Thereby, in the peroxide stage a greater increase in brightness can be achieved by a smaller peroxide consumption. Also the viscosity of the pulp is better maintained in the treatment.
A sufficiently acid treatment removes metals, but optimally chelation is carried out so that the removal of transition metals is as effective as possible, while alkaline earth metals Mg and Ca protecting the pulp and reducing the degradation of peroxide, will remain in the pulp. An optimal chelating pH is from 4 to 7. At higher pH the removal of all metals will decline, at lower pH also the protecting alkaline earth metals will be removed.